Bench grinders or table top tool grinders are used in practically every manufacturing or fabrication environment and sheet metal, casting, lock smithing, maintenance and tool manufacturing shops, and even in wood turning shops.
Traditional bench grinders use dual shaft AC motors and hot press composite bonded wheels.
Traditional tool cutters or grinders generally include a table top or base to which a grinding assembly with a motor and a composite bonded grinding wheel is mounted and either a single, adjustable cutting station, or multiple cutting stations for supporting the tool or cutter to be sharpened or ground at different angles relative to the grinding wheel.
Setups with composite grinding wheels require substantial mechanical effort to abrade or cut away material. Moreover, the particles on the composite wheels must erode for the wheel to stay sharp.
In composite material wheels, a significant portion of the material in the wheel is not of an abrasive nature, but is binder material. Consequently, much of the contact between the wheel and the article to be ground and pushed against the wheel is rubbing contact rather than cutting contact. In fact, 50% or more of the total contact area between the wheel and the article to be ground is in rubbing contact. This requires the wheels to be rotated at a certain threshold speed and the article to be forced against the wheel at a certain threshold force, before any usable rate of material removal can be achieved.
The actual speed of the edge of the stone wheel in a standard grinder wheel of 6 inch diameter is 61 mph (6 inch diameter=18 inch circumference or 1.5 feet×3600 rpm=5400 feet per minute=61 mph). However, the combination of elevated speed and contact force, together with the majority of the contact being rubbing type contact, creates several major disadvantages, the need for large, noisy motors, the generation of excessive heat, a slow material removal rate, wearing down of the grinding wheels, the creation of large amounts of dust (wheel wear) and the generation of smoke and unpleasant odors, due to (over)heating of the binder material in the wheel.
Composite “stone”/mineral composition wheels like aluminum oxide, carborundum, or silica type become instantly dull on the surface without constant attention or “dressing”, causing excess friction at these high speeds. Composite mineral wheels also are constantly changing shape, requiring the tool rests to be adjusted so as to maintain safe settings as they deteriorate, while emitting dust which is harmful to the operator. Also, the article being ground requires constant cooling down due to the frictional heat generated. The person skilled in the art of tool sharpening will appreciate that excessive heat is the most challenging problem with conventional grinder technology, since many tool materials are damaged by heat. This often requires the use of a coolant, adding another layer of complexity and impracticality.
The use of AC motors creates a further disadvantage, the need for a higher starting speed. It is a characteristic of AC motors that their rotation speed under load is significantly lower that at no load. This means that in order to operate at a desired speed under load, the motor speed without load must be adjusted significantly higher. However, due to the rubbing friction disadvantage of conventional grinder wheels, a high initial surface speed causes and even faster heating up of the article being ground and, thus, significantly increases the danger of overheating the article already at the beginning of the grinding operation. Moreover, although bench grinders and tool grinders with speed controlled AC motors are known, all AC motor speed controls, usually operating on the basis of phase clipping, result in progressively lower motor torque at decreasing speeds, making grinding at lower speeds extremely time consuming. Moreover, grinding speeds below 50% of the synchronous speed of the motor are impossible to achieve with AC motors, which means grinding below 1800 rpm will not be achievable with AC motor grinders.
Different types of composite or stone wheels are known. However, all wheels are subject to wear, especially uneven wear, which means the wheels have to be periodically dressed, causing unnecessary waste and even faster wearing down of the wheel. Thus, a need for an improved grinder exists.